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Fifteen miles southwest of Nairobi, Kenya, the Ngong Hills flatten into a vast savannah. The dusty terrain is speckled with thorn trees. Roaming cows nibble on whatever grass they can find.

Walking with the cows are nomadic herdsmen from the Masai tribe.

The Masai have long built their culture around cattle. They rely on the animals for food and use them as a measure of wealth and status. So, for the Masai, livestock diseases can be devastating.

Perhaps the most devastating of all was rinderpest ("cattle plague," in German), a disease that the United Nations says has now been eradicated. Rinderpest joins smallpox as the only diseases to be wiped off the face of the earth.

James Karnet Ole Tavo — a 70-year-old Masai herdsman with a lanky frame and looping earlobes — recalled a rinderpest outbreak in the 1960s. He said his animals first developed watery eyes and blisters around their mouths. A little later, they contracted severe diarrhea. Within a week, they were dead.

"Out of a hundred animals, you find that you're only left with 20 or 30," he said through a translator. "The rest are gone with rinderpest."

Daniel Gilfoyle, a historian at the British National Archives who has written extensively on rinderpest, called the illness "probably the most important single disease of domestic animals from an economic perspective."

The disease is caused by a virus related to measles. Gilfoyle said it first appeared centuries ago, in Central Asia.

"It spread very widely, throughout Asia, Southeast Asia" and to Europe, he said. "But it was in Africa that the disease probably had its biggest impact. People basically lost their money, lost their wealth, and were forced into wage labor."

Rinderpest killed more than 95 percent of the cattle in many parts of Africa in the 1890s. That deeply impoverished many communities and, according to Gilfoyle, made European colonization easier.

The disease also killed off huge numbers of African wildlife.

For centuries, people tried to fight rinderpest. As early as the 1700s, European farmers contained the spread of the disease by killing and burying sick animals.

In 1924, the World Organization for Animal Health was founded to coordinate a global campaign against the disease. In the late 1950s, British scientist Walter Plowright developed the first successful vaccine.

But it wasn't until 1994 that a new goal seemed feasible — total eradication.

"The science was right, the tools were there," said Mark Rweyemamu, former head of infectious diseases for the United Nations Food and Agriculture Organization (FAO). "Society and the governments wanted the job done."

FAO helped form a global rinderpest eradication program. Scientists distributed kits to test for the disease and told farmers to kill infected animals. Cows in surrounding areas were vaccinated.

The program worked. The last known case of rinderpest was discovered in Kenya in 2001, but for years no one wanted to declare the disease eradicated for fear that it would crop up again.

Only now, after a decade of surveillance, are scientists sure that the virus is gone.

"Now the only place where rinderpest virus exists," said Mark Rweyemamu, "is in the laboratories."

Back in Kenya, 70-year-old James Karnet Ole Tavo is glad to see rinderpest gone, but he said he doesn't really have time to be thankful. There are plenty of other animal diseases to worry about. On a recent day, his cows were being vaccinated against East Coast Fever.

Joe Kaka, a veterinarian who has worked on the outskirts of Nairobi for more than 50 years, said many locals used to be reluctant to have their cows immunized. They didn't trust the government.

"It's easier now," he said. The successful campaign against rinderpest has taught people to trust veterinary science.

Nearby, two veterinarians held a young calf. They gave it an injection and clipped a yellow tag to its ear. Then the Masai herdsmen collected their animals and headed back to the dusty plain.